The instabilities observed in the atomic cloud of a magneto-optical trap ar
e experimentally studied through the dynamics of the center of mass locatio
n and the cloud population. Two dynamical components are identified: a slow
, stochastic one affects both variables, and a fast, deterministic one affe
cts only the center of mass location. A one-dimensional stochastic model ta
king into account the shadow effect is developed from these observations an
d reproduces the experimental behavior. It is shown that instabilities are
driven by noise and present stochastic resonancelike characteristics.